This invention relates to concrete reinforced corrugated metal plate arch-type structures, such as used in overpass bridges, water conduits, or underpasses, capable of supporting large superimposed loads under shallow covers such as heavy vehicular traffic and more particularly a structure which may be substituted for standard concrete or steel beam structures.
Over the years, corrugated metal sheets or plates have proved themselves to be a durable, economical and versatile engineering material. Flexible arch-type structures made from corrugated metal plates have played an important part in the construction of culverts, storm sewers, subdrains, spillways, underpasses, conveyor conduits and service tunnels; for highways, railways, airports, municipalities, recreation areas, industrial parks, flood and conservation projects, water pollution abatement and many other programmes.
One of the main design challenges in respect of buried corrugated metal arch-type structure is that a relatively thin metal shell is required to resist relatively large loading around its perimeter such as lateral earth pressures, groundwater pressure, overburden pressure as well as other live and/or dead load over the structure. The capacity of such a structure in resisting perimeter loading is, apart from being a function of the strength of the surrounding soil, directly related to the corrugation profile and the thickness of the shell. While evenly distributed perimeter loads, such as earth and water pressures, generally would not create instability in an installed structure, the structure is more susceptible to uneven or localized loading conditions such as uneven earth pressure distribution during backfilling or live loads on the installed structure due to vehicular traffic. Uneven earth pressure distribution during the backfilling of the arch structure causes the structure to distort or peak, rendering the shape of the finished structure different from its intended most structurally sound shape. Live loads over the top of the structure, on the other hand, creates a localized loading condition which could cause failure in the roof portion of the structure.
A localized vertical load such as a live vehicular load imposed over an arch-type structure will create both bending stresses and axial stresses in the structure. Bending stresses are caused by the downward deformation of the roof thereby generating positive bending moments in the crown portion of the structure and negative bending moments near the hip portions of the structure. Axial stresses are compressive stresses caused by a component of the live load acting along the transverse cross-sectional fibre of the arch structure. In a buried metal arch structure design, the ratio of the bending stress to the axial stress experienced under a specific vertical load varies according to the thickness of the overburden. The thicker the overburden, the more distributed the vertical load becomes when it reaches the arch structure and the less bending the structure will be subjected to. The stress in an arch structure under a thick overburden is therefore primarily axial stress.
Corrugated metal sheets tend to fail more easily under bending than under axial compression. Conventional corrugated metal arch-type design deals with bending stresses created by live loads by increasing the overburden thickness, thereby disbursing the localized live loads over the thickness of the overburden and over a larger surface on the arch, the bending stresses on the arch is therefore minimized and the majority of the load is converted into axial forces. However, it is obvious that, by increasing the overburden thickness, the earth pressure on the structure is increased and stronger metal plates are therefore required. The need for a thick overburden also creates severe design limitations, such as limitation on the size of the clearance envelope under the structure or the angle of approach of a roadway over the structure. In a situation where the overburden thickness is limited and is shallow, the live load problem is traditionally solved by positioning an elongated stress relieving slab, usually made of reinforced concrete, near or immediately below the roadway extending above the area of shallow backfill. The elongated slab will act as a load spreading device so that localized vehicular loads will be distributed over a larger area on the metal arch surface. The problem with a stress relieving slab is that it requires on site fabrication thus involving additional fabrication time and substantial costs in labour and material. Moreover, in areas where concrete is not available, this is not a viable option.
Attempts have been made to strengthen a corrugated metal arch structure by the use of reinforcing ribs. In U.S. Pat. No. 4,141,666, reinforcing members are used on the outside of a box culvert to increase its load carrying capacity. The problem with that invention is that sections of the structure between the reinforcing ribs are considerably weaker than at the reinforcing ribs and hence, when loaded, there is a differential deflection or undulating effect along the length of the structure. To reduce this problem, longitudinal members are secured to the inside of the culvert to reduce undulation, particularly along the crown and base portions. It is apparent, however, that when these structures are used over stream beds or the like, it is not desirable to include inside the structure any attachments because of their tendency of being destroyed by ice flows and floods.
In U.S. Pat. No. 4,318,635, multiple arch-shape reinforcing ribs are applied to the interior/exterior of culverts to provide for reinforcement in the sides, crown and intermediate haunch or hip portions. Although such spaced apart reinforcing ribs enhance the strength of the structure to resist loads, they do not overcome the undulation problem in the structure and can add unnecessary weight to the structure by way of superfluous reinforcement. In addition to the above disadvantages, reinforcing ribs in this type of structure are often time consuming and complicated to install adversely affecting the costs of construction. Moreover, where relatively widely spaced rib stiffeners are used, structural design analyses become difficult for these structures. The discontinuity of the reinforcement and hence the variation in stiffness along the longitudinal length of a structure makes it difficult to develop the full plastic moment capacity of the section, thereby giving rise to a design that is generally unnecessarily conservative and uneconomical.
U.S. Pat. No. 3,508,406 by Fisher discloses a composite arch structure having a flexible corrugated metal shell with longitudinally extending concrete buttresses on either side of the structure. It is specifically taught that in the case of a wide spanning arch structure, the concrete buttresses may be connected with additional stiffening members extending over the top portion of the structure. Similarly, in U.S. Pat. No. 4,390,306 by the same inventor, an arch structure is taught wherein a stiffening and load distributing member is structurally fixed to the crown portion of the arch extending longitudinally for the majority of the length of the structure. It is also provided that the composite arch structure should preferably include longitudinally extending, load spreading buttresses on either side of the arch structure. The top longitudinal extending stiffener and buttresses can be made of concrete or metal and may even consist of sections of corrugated plate having its ridges extending in the length direction of the culvert.
In the Fisher patents, continuous reinforcement is provided along the structure by means of the crown stiffener and the buttresses. The buttresses are designed to provide stability to the flexible structure during the installation stage, that is, before the structure is being entirely buried and supported by the backfill. They provide lengths of consolidated material at locations to resist distortion when compaction and backfilling equipment is used, enabling the backfilling procedure to continue without upsetting the structure""s shape. The top stiffener with internal steel reinforcing bars acts to weigh down the top part of the structure to prevent it from peaking during the early stages of backfilling and compaction and as a load spreading device that helps distribute the vertical loads on the structure, thus reducing the minimum overburden requirement. The top stiffener in the length direction of the structure rigidities the top portion of the arch by using shear studs to structurally connect the concrete beam to the steel arch to provide for positive bending resistance in the arch top. This multi-component stiffener moves towards a structure which permits the use of reduced overburden but cannot provide for a large reduction in overburden thickness or for very large spans in arch design. The primarily reason is that the top stiffener in Fisher is not designed to resist negative bending moments typically found in the hip portions of shallow cover arches and wide spanning arches. The purpose of the spaced apart transverse members between the top stiffener and the side buttresses is to provide some rigidity to the structure to prevent distortion during the backfilling stage. They are not members designed to resist negative moments. Further, while an installed flexible arch structure is subject to positive bending moments at the crown under live load conditions, it is subject to negative bending moments at the same location during backfilling when it is being pressured from the sides and the top will distort by way of peaking. The top stiffener in Fisher, while it is designed to take advantage of a shear-bond connection between the concrete and steel to resist positive bending moments in the top portion of the arch, negative bending moments in the same region during backfilling are resisted simply by the provision of reinforcing bars in the upper part of the concrete slab, thus requiring in-situ forming and re-bar work, adversely affecting construction costs. Also, since the top stiffener and side buttresses are of significant sizes, the weight of the completed structure is substantially increased.
In Sivachenko, U.S. Pat. No. 4,186,541, a method of forming corrugated steel plates from flat plate stock for use in constructing, inter alia, metal arch structures is disclosed. Specific reference was made to the additional strength advantage of a double corrugated plate configuration wherein plates are joined together along opposite troughs either directly or with spacers between them. It is noted that the double plate assembly may be left hollow or may be filled with concrete or a like material. The concrete between the plates may be reinforced with conventional reinforcing steel bars which may be oriented parallel or transversely to the corrugations of the plates. It is apparent that when concrete is placed between the plates without reinforcement, it will only act as a filler and will not enhance the strength characteristics of the assembly. Even when the concrete is provided with reinforcing bars, the re-bars are not designed for shear-bond connection between the concrete and the corrugate steel plates and when the assembly is subject to bending, the concrete and steel plates function independently of one another. That system moves towards a method of stiffening a corrugated metal plate structure by the use of a double plate assembly with a concrete-filled centre typical of a sandwich-type support structure. In the case of a burried arch structure with multiple curves, the installation of re-bars in accordance with Sivachenko will become an even more difficult task.
In U.S. Pat. No. 5,326,191 continuous corrugated metal sheet reinforcement is secured to at least the crown of the culvert extending continuously over the length of the culvert. This culvert design solves the problem associated with prior art spaced apart transverse reinforcement and is inherently capable of resisting both positive and negative bending moments. However, continuous reinforcement on large span structures can become cost prohibitive and difficult to install.
The concrete reinforced corrugated metal arch-type structure of this invention overcomes a number of the above problems. The composite concrete metal beams, as provided by this invention enhance the structure""s resistance to both positive and negative bending moments induced in the structure by virtue of either shallow overburden supporting live heavy load vehicular traffic or during backfilling of the arch-type structure. Each continuous concrete filled cavity defined by interconnecting an upper plate and a lower corrugated plate of this invention will act as a composite metal encased concrete beam functioning as a curved beam column stiffener with, bending moment and axial load capacities to provide for greater design flexibility in providing arch structures with shallow overburden.
According to an aspect of the invention, a composite concrete reinforced corrugated metal arch-type structure comprises:
i) a first set of shaped corrugated metal plates interconnected in a manner to define a base arch structure of a defined span cross-section, height and longitudinal length, the base arch having a crown section and adjoining hip sections for the span cross-section and corrugated metal plates of defined thickness having corrugations extending transversely of the longitudinal length of the arch to provide a plurality of curved beam columns in the arch;
ii) a second series of shaped metal plates interconnected in a manner to overlay the first set of interconnected plates of the base arch, the second series of plates extending continuously in the transverse direction to include at least the arch crown;
iii) the interconnected series of second plates and the first set of plates defining at least one individual, transversely extending, enclosed continuous cavity, each cavity being defined by an interior surface of the first set of plates and an opposing interior surface of the second series of plates;
iv) concrete filling the continuous cavity from cavity end to end as defined by the transverse extent of the second series of plates, the concrete filled cavity defining an interface of the concrete enclosed by the metal interior surfaces of the interconnected second series of plates and first set of plates;
v) the interior surfaces of the cavity for each of the first and second plates having separate means for providing shear bond at the concrete-metal interface to provide a plurality of curved beam column stiffeners to enhance combined positive and negative bending resistance and axial load resistance of the base arch structure, there being a sufficient number of the second series of plates to provide a sufficient number of the curved beam column stiffeners to support anticipated loads imposed on the structure.